This invention relates to an improved method for fabricating a highly pure, crystalline semiconductor material having stoichiometric proportions. More particularly, this invention concerns itself with an improved method for producing highly pure, stoichiometric indium phosphide crystalline materials (which are especially useful as substrates for epitaxy procedures) in a rapid manner as compared with past procedures.
Indium phosphide, a group III-V semiconductor compound, has been found to be especially useful in two areas of technical interest. Its large band gap (1.35 eV) and high electron mobility make it useful as a semiconductor material, especially when employed in its highly pure form, and as a crystalline substrate for device fabrication by epitaxial deposition techniques. Thin film InP devices have great potential for use in integrated optic applications and high frequency microwave devices. However, it is extremely difficult to synthesize indium phosphide to the degree of purity necessary for its successful utilization as a thin film device.
A number of methods have been suggested for synthesizing indium phosphide. One method for growing fairly large crystals involves the direct reaction of elemental phosphorus with elemental indium. This method offers the advantage of producing a relatively pure product since there is no possibility of contamination by other reactants. Unfortunately, the reaction often leads to a large pressure buildup with a resulting explosive potential. This method requires small, strongly sealed containers or bombs and is an expensive and dangerous method for conducting the reaction. Reaction methods involving compounds of indium and phosphorus, rather than elemental reactants, have also been suggested, but the resulting indium phosphide has often been lacking in the degree of purity needed to grow good single crystals.
Another method which has proven successful in producing a highly pure product involves transporting phosphorus in the vapor phase from a source of molten red phosphorus in a first zone of a reaction vessel to a source of molten indium positioned in a second zone of the vessel which is thermally isolated from the first zone. This allows a reaction to occur at a controlled rate without overheating the phosphorus. Although this method provides a highly pure product, it nevertheless still suffers from a pressure control standpoint in which explosions often occur during heating and cooling of the reaction vessel. A quartz ampoule is generally utilized as the reaction vessel and it was found that indium rich indium phosphide wetted and cracked the quartz. Then, the resulting high pressure of the phosphorus would cause the cracked quartz ampoule to explode.
In order to counter the high pressure buildup in the ampoule, the ampoule and furnace can be placed inside of a pressure chamber. This requires that the pressure inside the ampoule be balanced to that in the chamber during heating and cooling. A pressure balancing technique using a quartz bellows to measure the pressure differential across the wall of the ampoule was found to be too complicated and difficult to use reproducibly. High pressure synthesis of volatile chemicals is complicated by the large heat transfer coefficient of the high pressure gas in the pressure chamber. Passive temperature/pressure control by thick layers of insulation was found to be ineffective and explosion of the ampoules was inevitable.